1. Field of the Invention
The present invention relates to a process for the fabrication of an inertial sensor with failure threshold.
2. Description of the Related Art
As is known, modern techniques of micromachining of semiconductors can be advantageously exploited for making various extremely sensitive and precise sensors, having further small overall dimensions. The so-called MEMS sensors (or micro-electro-mechanical-system sensors), are sensors that can be integrated in a semiconductor chip and are suitable for detecting various quantities. In particular, both linear and rotational MEMS accelerometers with capacitive unbalancing are known. In brief, these accelerometers are normally provided with a fixed body and of a mobile mass, both of which are conductive and are capacitively coupled together. In addition, the capacitance present between the fixed body and the mobile mass may vary, and its value depends upon the relative position of the mobile mass with respect to the fixed body. When the accelerometer is subjected to a stress, the mobile mass is displaced with respect to the fixed body and causes a variation in the coupling capacitance, which is detected by a special sensing circuit.
As mentioned previously, MEMS accelerometers are extremely sensitive and precise; however, they are not suitable for being used in many applications, mainly because they are complex to make and their cost is very high. On the one hand, in fact, the processes of fabrication involve the execution of numerous non-standard steps and/or the use of non-standard substrates (for example, SOI substrates); on the other hand, it is normally necessary to provide feedback sensing circuits based upon differential charge amplifiers, the design of which frequently involves some difficulties.
In addition, in many cases the precision of capacitive MEMS sensors is not required and, indeed, it is not even necessary to have an instantaneous measurement of the value of acceleration. On the contrary, it is frequently just necessary to verify whether a device incorporating the accelerometer has undergone accelerations higher than a pre-set threshold, normally on account of impact. For example, the majority of electronic devices commonly used, such as cell phones, are protected by a warranty, which, however, is no longer valid if any malfunctioning is due not to defects of fabrication but to an impact consequent on the device being dropped onto an unyielding surface or in any case on a use that is not in conformance with the instructions. Unless visible damage is found, such as marks on the casing or breaking of some parts, it is practically impossible to demonstrate that the device has suffered damage that invalidates the warranty. On the other hand, portable devices, such as cell phones, exactly, are particularly exposed to being dropped and consequently to getting broken, precisely on account of how they are used.
Events of the above type could be easily detected by an inertial sensor, which is able to record accelerations higher than a pre-set threshold. However, the use of MEMS accelerometers of a capacitive type in these cases would evidently lead to excessive costs. It would thus be desirable to have available sensors that can be made using techniques of micromachining of semiconductors, consequently having overall dimensions comparable to those of capacitive MEMS sensors, but simpler as regards both the structure of the sensor and the sensing circuit. In addition, also the processes of fabrication should be, as a whole, simple and inexpensive.